1. Field of Invention
The present invention relates to a spin transistor and a manufacturing method thereof, and more particularly, to a spin transistor of miniature structure and a manufacturing method thereof.
2. Related Art
The conventional transistor is a semiconductor element utilizing the transmitting effect of two carrier-electrons and holes, and is the core element of modern microelectronic circuitry. Digital circuits and analog circuits with various different functions are all assembled with transistors. A spin transistor controls electric signals of elements by making use of the spin upward and spin downward properties of electrons to generate an effect similar to that of a conventional transistor. A spin transistor as currently developed mainly interposes a layer of magnetoresistive element, used as an electron spin valve, between two potential energy barriers, and utilizes different configurations of magnetic arrangement of ferromagnetic layers in the magnetoresistive element to control the flow of thermal electrons.
A structure of the spin transistor as proposed by Monsma et al. in 1995 uses two semiconductor silicon substrates to sandwich a layer of magnetic multi-layer film, which is made by a vacuum bounding technique. This magnetic multi-layer film is formed of metal layers of Co, Cu, Co, Pt, etc., and is used as an electron spin valve and defined as a base. The silicon substrate bounding with Pt is used as an emitter, forming a Schottky barrier between Pt and the silicon substrate; the silicon substrate bounding with Co is used as a collector, forming another Schottky barrier between Co and the silicon substrate. Electrically, the Schottky barrier of the emitter is of negative bias, and the Schottky barrier of the collector is of positive bias, thus the emitter can accelerate electrons to pass through the Schottky barrier, enter the magnetic multi-layer film (the base), and become thermal electrons. The amount of thermal electrons passing through the base depends on whether the magnetic directions of two Co layers in the spin valve are the same. If the magnetic directions are opposite, the amount of thermal electrons passing through the base is small. If the magnetic directions are parallel, the amount of thermal electrons passing through the base is large. However, it is not easy to achieve miniaturization in production by the vacuum bounding technique and the cost is high.
Another spin transistor structure with two potential energy barriers is proposed by Mizushima et al. in 1997, wherein a magnetoresistive element is made as the base on an n-type GaAs substrate as the collector. One potential energy barrier is formed between the base and the aluminum oxide formed by oxidation of aluminum. The aluminum oxide is plated with metal as the emitter, which provides a Schottky barrier as another potential energy barrier. However, the pattern of this element is defined by a contact mask, thus it also is not easy to miniaturize.
Concerning the manufacturing method of patterns, contact mask lithography involves placing a contact mask with patterns directly on a substrate. The contact mask is used to directly block the portions not required when plating the film. Yellow light lithography is the most commonly used pattern transfer technique in semiconductor processing and micro-electromechanical processing, and the basic principle thereof is to: transfer the patterns on a mask onto the photosensitive layer (i.e. photoresist) on the chip surface in the way of UV light exposure, then remove the places not required by a special etching solution (developer), thus a photoresist structure with desired patterns is obtained on the chip surface. R. Sato et al. in 2001 and Sebastiaan van Dijken et al. in 2003 and 2005 all suggested that, with the method of defining patterns by a contact mask, the minimum line width of a pattern is 100 micron. In addition, O. M. J. van't Erve et al. in 2002 suggested that, with the method of defining patterns by yellow light lithography, the minimum line width of a pattern is 350 micron. In summary, no matter which process is used, the size of an element is bigger than 100 micron, which hampers the popularization and application of spin transistors. Therefore, reducing the element size and simplifying the manufacturing process of spin transistors is indeed an important problem.
Electron beam lithography is one of the most important techniques for producing a structure of sub-micrometer to nanometer scale, the basic principle thereof being to directly write a special photoresist by electrons. The electrons are accelerated by high voltage and the traveling paths of the electrons are controlled by electromagnetic coils, thus various periodic and non-periodic patterns can be produced. Since the wavelength of electrons is smaller than that of the light source used by general photolithography, a higher resolution is provided. The electron beam lithography may easily achieve a line width with the size of hundreds of nanometers to several nanometers. It not only may be used to produce the mask required by photolithography, but also perform the direct write, that is, patterns are defined without a mask. Currently, the electron beam lithography has not yet been applied for defining patterns for the emitters, collectors, and bases of spin transistors.
To summarize the aforementioned content, a spin transistor as currently developed mainly interposes a layer of magetoresistive element between two potential energy barriers. The manufacturing process is complicated, and it is difficult to miniaturize the spin transistor due to the limitation of the lithography processes.